Summary

The rhythmic heart beat is coordinated by electrical impulses transmitted from Purkinje fibers of the cardiac conduction system. During embryogenesis, the impulse-conducting cells differentiate from cardiac myocytes in direct association with the developing endocardium and coronary arteries, but not with the venous system. This conversion of myocytes into Purkinje fibers requires a paracrine interaction with blood vessels in vivo, and can be induced in vitro by exposing embryonic myocytes to endothelin-1 (ET-1), an endothelial cell-associated paracrine factor. These results suggest that an endothelial cell-derived signal is capable of inducing juxtaposed myocytes to differentiate into Purkinje fibers. It remains unexplained how Purkinje fiber recruitment is restricted to subendocardial and periarterial sites but not those juxtaposed to veins. Here we show that while the ET-receptor is expressed throughout the embryonic myocardium, introduction of the ET-1 precursor (preproET-1) in the embryonic myocardium is not sufficient to induce myocytes to differentiate into conducting cells. ET converting enzyme-1 (ECE-1), however, is expressed preferentially in endothelial cells of the endocardium and coronary arteries where Purkinje fiber recruitment takes place. Retroviral-mediated coexpression of both preproET-1 and ECE-1 in the embryonic myocardium induces myocytes to express Purkinje fiber markers ectopically and precociously. These results suggest that expression of ECE-1 plays a key role in defining an active site of ET signaling in the heart, thereby determining the timing and location of Purkinje fiber differentiation within the embryonic myocardium.

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